User @rds has given the correct answer about how to fix this problem, but I can tell you that this issue happens mainly after stopping Google Services Framework (this is a practice that some people recommend in order to be able to get OTAs when delayed to be available for your phone).

I had Android 4.2, but I was trying to get a recently released bug-fixing 4.2.1 update, so I just did this (stop Google Services Framework), but after updating Android and restarting my smartphone, I got that infamous [RPC:S-5:AEC-0] error when trying to get some updates for my apps.

First I tried, unsuccessfully, what @Anthony says. Then I did what @rds says (removing/adding my Google account) and the problem has gone!

Let me add: this is not the first time I get that error, but it is until now that I realize both the reason and the solution, so I'm sharing this with you.

The server error RPC:S-5:AEC-0 can be worked around by going to the 1Mobile App Market and downloading free apps only via that service. Not ideal, but at least it's enabled me to get BBC News, BBC Media Player and Skype.

I've done a fair bit of searching and just can't find an answer for this server error problem.

added my second Google account. I then went to the Play Store > [MENU] Accounts and selected the new account I had added, followed by switching back to the previous account. This resolved the issue for me.

I have this issue with my tablet and found that saving all my important info and factory resetting the tablet. once I had done this I set up a new g-mail account. so basically starting a fresh. this worked and did not show the error msg any more (RPC:S-5:AEC-O). HOPE THIS HELPS